PROJECT SUMMARY/ABSTRACT Rotator cuff tears are the most common cause of shoulder pain and related disability. Likelihood of injury increases dramatically with advancing age. Over time following injury, the tendon retracts, leaving a large defect. However, not only is the tissue gap a problem, but also the nature of the muscle changes, with an end result of decreased compliance and increased stiffness. Though it is clear that the tension required for repair increases with time after injury, there is much individual variation. Thus, it is critical to have an individualized pre-surgical prediction of extensibility of rotator cuff muscles assist in pre-surgical planning and monitoring of recovery throughout the rehabilitation process. Our long-term goal is to develop a non-invasive tool for acquiring such information to improve the planning, and thus, the outcome of treatment. The purpose of this study is to initiate the development by adopting a novel technology, namely, Shear Wave Elastography (SWE). The SWE provides quantitative in vivo measurements of tissue stiffness and viscosity by evaluating shear wave propagation speed, which is inherently related to tissue mechanical properties. We propose this unique and innovative approach which combines b-mode ultrasound imaging (to assess muscle geometric and morphological properties) with SWE (to assess muscle material properties) to quantify muscle structural properties which will then be used to predict the extensibility of rotator cuff muscles (typically supraspinatus muscle). The objective of this study will be accomplished in the following three specific aims using cadaver shoulder specimens in vitro: Specific Aim 1: To obtain the material and geometric properties of the supraspinatus muscles using SWE and b-mode ultrasound imaging, respectively. Specific Aim 2: To measure muscle extensibility and structural properties of the supraspinatus muscles through mechanical testing. Specific Aim 3: To establish the relationship between structural stiffness of in vitro supraspinatus muscles (measured in Specific Aim 2) with muscle modulus measured with SWE and geometric/morphological properties based on b-mode imaging (measurements in aim 1) via regression analyses. The contribution will be significant because it will provide a necessary, non-invasive assessment of whole-tissue extensibility that will directly aid in prognosis, pre-surgical planning, and post-surgical evaluation of rotator cuff repair, one of the most common and challenging procedures in orthopedics.